In recent years, as various data such as image data and audio data are digitalized, amount of digital data is dramatically increased. Based on this, an optical disc drive suitable for realizing of high-capacity and high density has been developed.
Meanwhile, because copyright protection is carried out with respect to many image data and audio data, needed is a way how these copyright works are protected from being copied without authorization. In order to carry out the copyright protection, the techniques, in which sub data for copyright protection, such as encryption key, is recorded on an optical disc or the like, has been developed.
Among these techniques, for example, Japanese Laid-Open Patent Application Tokukai 2000-195049/2000 (published on Jul. 14, 2000; hereinafter referred to as “patent document 1”), and Japanese Laid-Open Patent Application Tokukai 2002-203374/2002 (published on Jul. 19, 2002; hereinafter, referred to as “patent document 2”) disclose a technique in which superimposition of sub data is carried out in accordance with minute displacement of pits by which main data such as image data is recorded. Here, the technique disclosed in patent document 1 is described.
FIG. 12 shows how the pits are formed on an optical disc in accordance with the technique disclosed in patent document 1. The technique employs a pit length of each pit, and a distance between the pits to record encrypted audio data. Further, in the technique, each pit P is formed so that the minute displacement, toward outer or inner periphery of a track center TC in a radial direction, is provided for each pit P. The minute displacement allows encryption key data KY to be recorded for decrypting the encrypted audio data.
FIG. 13 shows a block diagram of an optical disc drive 130, in which a reproduction is carried out with respect to an optical disc on which the audio data and the encryption key data KY is thus recorded.
In the optical disc drive 130, a servo circuit 133 controls a spindle motor 132 so that an optical disc 131 is driven and rotated at a constant linear velocity.
Further, an optical head 134 irradiates a laser beam to the optical disc 131. Then, the optical head 134 receives the light reflected from the optical disc 131 via a predetermined light receiving device, and outputs a radio frequency signal RF whose signal level varies depending on the light intensity of the reflected light on a light receiving surface of the light receiving device. This allows the signal level of the radio frequency signal RF to vary in accordance with the pits recorded on the optical disc 131.
Further, the optical head 134 processes a reception result of the reflected light by using the push-pull method. This allows the optical head 134 to generate a push-pull signal PP, whose signal level varies in accordance with a location of a pit with respect to a location of an irradiated laser beam, in the radial direction of the optical disc 131. Further, the optical head 134 generates and outputs a focus error signal whose signal level varies in accordance with amount of focus error.
The servo circuit 133 band-limits the push-pull signal PP so as to generate a tracking error signal, whose signal level varies in accordance with de-track amount. The de-track amount indicates how far a location of irradiated laser beam is with respect to the track center. With the tracking error signal, the servo circuit 133 carries out a tracking control to the optical head 134. Further, the servo circuit 133 carries out a focus control with respect to the optical head 34 in accordance with the focus error signal.
A high-pass filter (HPF) 135 suppresses low-frequency components of the push-pull signal PP so as to remove the de-track amount component from the push-pull signal PP, whose signal level varies in accordance with a location of a pit with respect to a location of an irradiated laser beam, the de-track amount component being indicative of how far a location of irradiated laser beam is with respect to the track center. This allows the high-pass filter 135 to detect a displacement detection signal HPP, whose signal level varies in accordance with the location of the pit with respect to the track center.
The radio frequency signal RF is binarized by a binarization circuit 136, based on a predetermined reference signal level, so as to generate a binary signal BD.
A PLL circuit 137 operates in synchronization with the binary signal BD, so as to reproduce a channel clock CCK of the radio frequency signal RF.
The binary signal BD is sequentially latched by an EFM (Eight to Fourteen Modulation) decoder circuit 138 in synchronization with the channel clock CCK. This allows the EFM decoder circuit 138 to reproduce a reproduction data that corresponds to an EFM encoding signal S2. Further, the EFM decoder circuit 138 carries out an EFM decoding of the reproduction data, and then comparts the decoded data per 8 bits based on frame sync. Thereafter, the EFM decoder circuit 138 de-interleaves 8-bit signal thus comparted, and sends it to an ECC (Error Correction Code) circuit 139.
The ECC circuit 139 carries out an error correction with respect to data, sent from the EFM decoder circuit 138, in accordance with an error correction code which is added to such data. This allows the encrypted audio data to be reproduced and outputted.
The encrypted audio data is decrypted by a decryption circuit 140 with the use of an encryption key data KY, which has been detected by a key detection circuit 142, and then the audio data thus decrypted (audio data D1) is outputted from decryption circuit 140.
The decryption circuit 140 thus outputs the audio data D1 to a digital/analog (D/A) converter circuit 141, in which a digital/analog conversion is carried out with respect to the audio data D1, so that an analog audio signal S4 thus converted is outputted from the D/A converter circuit 141.
The key detection circuit 142 processes the displacement detection signal HPP, in accordance with the channel clock CCK and the binary signal BD, respectively, so as to reproduce the encryption key data KY. Then, the encryption key data KY thus reproduced is sent to the decryption circuit 140.
Here, the displacement detection signal HPP obtained from each of the pits has significantly a deteriorated SN (signal-to-noise) ratio because each displacement of the pits is minute.
In view of the circumstances, the key detection circuit 142 accumulates each frame of the displacement detection signals HPP corresponding to one frame so as to carry out binary discrimination. On this account, the discrimination allows the encryption key data KY to be reproduced because high SN ratio is maintained. Accordingly, the encryption key data KY, which is recorded on the optical disc 131 so as not to be found out, is securely reproduced.
In the technique of patent document 1, when the optical disc drive 130 reproduces the optical disc 131, the signal qualities of the push-pull signal PP and the displacement detection signal HPP change due to a disturbance such as an unfocussed optical head 134, and/or a tilt of the optical disc 131.
When the disturbance is large, the signal quality of the displacement detection signal HPP deteriorates. Accordingly, the accumulation processing should be carried out for a long time to obtain adequate SN ratio in the displacement detection signal HPP. On the other hand, when the disturbance is small, the signal quality of the shift detection signal HPP is good. Therefore, it is possible to obtain a displacement detection signal HPP while maintaining sufficient SN ratio, even if the accumulation processing is carried out for a relatively short time.
It should be appreciated that the encryption key data KY is essential for decryption of the encrypted audio data. Therefore, the encryption key data KY always needs to be accurately obtained.
On this account, in the technique of patent document 1, it appears that the accumulation processing is carried out for such a sufficient long time that corresponds to one frame, so as to obtain an accurate encryption key data KY even in a state of a possible worst signal quality during normal reproducing.